Even though both are allowed in the style guide, it recommends to use
'using', as its syntax is more consistent with the rest of C++.
This CL turns all typedefs in src/snapshot to 'using' declarations.

R=jgruber@chromium.org

Bug: v8:8834
Change-Id: Ie555e9ac7e1ec04c20d411647f8ab70f671c1cb2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1545903
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60699}